2.77a - Keep foot on the ground with anchor OR do not allow any vertex below certain world Z
well, you can place a bone(anchor) in the desired location on the floor, then add a copy location and copy rotation constraints to the feet targetint the bone(anchor). then when the influence of the constraints is 1.0 the foot will be stick to the floor(the anchor bone position/rotation). you will have to keyframe the influence value that can be easy adding a driver for it.